1. Field of the Invention
This invention relates to a vehicle lamp having a plastic lens and more particularly to a method for surface treatment of its lens.
2. Description of the Related Art
Transparent lenses or equivalents are now employed in vehicle lamps such as headlamps. However, such a lens becomes fogged with only a few waterdrops sticking to its inner surface and this poses a serious problem in view of its external appearance as quality. Consequently, it has heretofore been contrived to obtain a lens whose inner surface is formed with an antifogging coating film.
In recent years, plastic lenses are increasingly adopted in vehicle lamps such as headlamps. This is because the plastic lenses are lightweight and excellent in shock resistance in comparison with glass lenses; however, as the plastic lenses are poor in scratch resistance, weather resistance and solvent resistance, hard coating films are often formed on their outer surfaces.
From the fact stated above, it is desirable to form antifogging coating films on the inner surfaces and hard coating films on the outer surfaces of transparent lenses or equivalents made of plastic that are adopted as vehicle lamps.
However, the following problem will be posed when it is attempted to form the antifogging coating film and the hard coating film in an offhand manner with the adoption of the aforementioned arrangement.
More specifically, when the hard coating film or the antifogging coating film is formed, a backing jig is employed in order to support a lens. However, as shown in FIG. 8, if the hard coat film is formed after the antifogging coating film is formed, there are high possibility that that a lens 6 is scratched because the backing jig 4 may come in contact with the outer surface 6a of the lens 6.
An object of the present invention made in view of the aforementioned circumstances is to provide a method for surface treatment of a lens of a vehicle lamp so that an antifogging coating film may be formed without scratching the lens when the antifogging coating film is formed on the inner surface of the plastic lens when a hard coating film is formed on the outer surface thereof.
The present invention is intended to accomplish the object above by implementing a method for surface treatment of a lens through the steps carried out in the following proper order.
The method according to the present invention comprises forming a hard coating film on a outer surface of the lens by heating to harden after the hard coating film is applied onto the outer surface of the lens; cooling the lens formed with the hard coating film until the inner surface of the lens has a predetermined temperature; and forming a antifogging coating film on a inner surface of the lens by heating to dry after the antifogging coating film is applied onto the inner surface of the lens.
The xe2x80x9cantifogging coating filmxe2x80x9d may be formed on the whole or part of the inner surface of the lens as long as the film forming range includes a target range where the lens may become fogged.
The xe2x80x9chard coating filmxe2x80x9d may be formed on the whole or part of the outer surface of the lens as long as the film forming range includes a target range where the scratching of the lens and weather and solvent resistance should be avoided.
As shown in the arrangement above, since the hard coating film is formed before the antifogging coating film is formed according to the present invention, the presence of the hard coating film can prevent the lens from being scratched even though a backing jig is brought into contact with the outer surface of the lens when the antifogging coating film is formed.
Therefore, according to the present invention, the antifogging coating film is formable without scratching the lens even in a case where the antifogging coating film is formed on the inner surface and the hard coating film is formed on the outer surface of the plastic lens.
As the antifogging coating film is formed on the inner surface of the lens according to the present invention, even if a transparent lens or an equivalent is used, the surface-active action of the antifogging coating film prevents poor external appearance that waterdrops sticks to the inner surface of the lens and becomes a water film so that the lens fogs up. On the other hand, the formation of the hard coating film on the outer surface of the lens can improve scratch resistance, weather resistance and solvent resistance of the lens.
Lens elements may or may not be formed on the inner surface of the xe2x80x9clens.xe2x80x9d In the latter case, however, as the lens looks foggy even with a few waterdrops sticking thereto, the adoption of the arrangement according to the present invention is specifically effective.
Although the steps of forming the hard coating film, cooling the lens and forming the antifogging coating film constituting a method for surface treatment of the lens may be conducted in different places, the following working effect is achievable by carrying out the whole step in the same clean.
When the aforementioned steps are performed in entirely different places, the number of physical distribution man-hours would be increased to that extent and while the lens formed with the hard coating film is conveyed up to a place where the antifogging coating film is formed, there is the possibility that the lens may be soiled with dirt or scratched. Moreover, the moisture in the air is absorbed by the lens before the antifogging coating film is formed and a whitening phenomenon may occur during the step of forming the antifogging coating film. In other words, since the antifogging coating is hydrophilic, the moisture absorbed into the lens is introduced into the antifogging coating film in the form of waterdrops immediately after the application of the coating and then the moisture in the antifogging coating film is caused to evaporate by heat-drying, so that traces of waterdrops remain to exist. Consequently, there may occur the whitening phenomenon in that the antifogging coating film looks whitish.
On the contrary, performing the whole step mentioned above within the same clean room not only reduces the number of man-hours but also prevents the lens from being soiled and scratched during the conveyance of the lens and also prevents the whitening phenomenon of the antifogging coating film from occurring.
With the arrangement above, the xe2x80x9cpredetermined temperaturexe2x80x9d is not restricted to a specific one as long as it is lower than the temperature immediately after hardening by heating during the step of forming the hard coating film. However, setting the inner surface of the lens at temperatures higher than the room temperature of the clean room makes possible the application of the antifogging coating film while the dried condition of the lens is maintained after the heat-hardening process during the step of forming the hard coating film. It is thus prevented to allow the occurrence of the whitening phenomenon in which the lens absorbs the moisture in the air before the coating is applied.
On the other hand, setting the xe2x80x9cpredetermined temperaturexe2x80x9d too high causes a phenomenon in which the antifogging coating film is not uniformly formed over the whole area of the antifogging coating film forming area on the inner surface of the lens because the solvent content in the coating is volatilized previously after the antifogging coating film is applied (unsatisfactory leveling). Consequently, it is preferred to set the predetermined temperature at 35xc2x0 C. or lower.
A method of cooling the lens up to the xe2x80x9cpredetermined temperaturexe2x80x9d during the aforementioned cooling step is not restricted in particular but may be what is adoptable in cooling the lens by forcibly blowing cooled air or air thereagainst or by natural cooling depending on the working conditions.
Incidentally, the thickness of the antifogging coating film is also not restricted in particular but may preferably be set at 10 xcexcm or less for the reason stated below.
Although the antifogging coating film demonstrates its antifogging function whether it is thin or thick, making it too thick needs to apply the coating a plurality of times or prolong the coating discharge time and this results in increasing working time and cost for extra coating, preventing the coating from dripping or necessitating long coating crosslinking time. On the contrary, setting the thickness of the antifogging coating film at 10 xcexcm or less could solve these problems completely or to some extent.
The time interval between the completion of application of the coating and the commencement of the heat-drying process during the step of forming the antifogging coating film may preferably be set shorter in view of preventing the moisture in the air from being absorbed into the antifogging coating film on one hand and a certain length of time should be secured to obtain a uniform antifogging coating film on the other; in other words, well-balanced time may be set in consideration of both cases. However, the moisture is readily absorbed into the antifogging coating film as the humidity increases; therefore, it is preferred to shorten the time interval between the completion of application of the coating and the commencement of the heat-drying process stepwise for correcting purposes in response to a rise in temperature within the clean room.
With arrangement above, the heat-drying temperature during the step of forming the antifogging coating film is not restricted in particular but if it is set too high, the difference in thermal expansion coefficient between the hard coating film and the lens may cause the hard coating film to crack, whereas if it is set too low, the antifogging coating film is insufficiently hardened as the coating is not crosslinked, so that its resistance to humidity is not sufficiently secured. Therefore, it is preferred to set the heat-drying temperature at 110-130xc2x0 C. during the step of forming the antifogging coating film.